It is well known in the orthodontic profession that plastic orthodontic brackets are a desirable choice for an orthodontist for orthodontically treating a patient where aesthetics is of paramount interest as plastic brackets can be easily molded of clear or translucent plastic resins that enhance the aesthetics of a bracket system, which are particularly better than that of metal brackets. However, plastic brackets have drawbacks as far as strength characteristics are concerned, and also as far as methodology for bonding the brackets to teeth.
To overcome the strength factor, plastic brackets have been filled with reinforcing substances, such as described in U.S. Pat. Nos. 3,922,787 and 4,107,844, and provided with metal archwire slots, as also shown in U.S. Pat. No. 4,107,844.
With respect to bonding plastic brackets to teeth, primers are first used such as methacrylate-based liquid for softening the tooth-attaching surface of the bracket prior to using "one step" or paste--paste bonding composites. Difficulties in using primers often yield unsatisfactory bonding results. Primers are temperature dependent which yield more or less final bond strength. Primer coating thickness affects bond strength and presently can only be applied at chairside by eyeballing the thickness. Excess primer on the base of the bracket can flow into tie wing areas and inadvertently soften critical areas of the bracket leading to premature bracket failure. Primers are shelf-life sensitive and can lose potency which cannot be detected during use but may be exhibited later in excess bracket bond failure.
Where a plastic bracket includes a strength-reinforcing filler, the filler may diminish the primer reaction sites on the bracket base which ultimately could affect bond strength. Most plastic brackets are pure polycarbonate or glass-filled polycarbonate. While glass-filled polycarbonate brackets produce better strength characteristics in a finished bracket, bonding becomes more difficult with higher filled percentages as the glass filler competes with reaction sites on the base, yielding less bond strength. On the other hand, where less filler is used, a weaker bracket is produced.
As above mentioned, only "one step" or paste--paste bond composites are now generally used for bonding plastic brackets with any degree of reliability. Visible light-cured composites, which are rapidly gaining wide acceptance in the orthodontic field, cannot be effectively used due to extreme variability of bond strength, and/or significantly greater chair time needed for bracket base preparation.
Various base structures have previously been suggested for enhancing the mounting of plastic brackets onto teeth. For example, the bracket in U.S. Pat. No. 3,303,565 shows in one embodiment a curled lip for defining a channel designed to overlie the incisal edge of a tooth.
The bracket in U.S. Pat. No. 3,765,091 shows perforation of the base in the embodiment of FIG. 1 and the use of slots in the embodiment of FIG. 5, and the use of blind openings in the embodiment of FIG. 11. Other types of slotting arrangements have also been used. However, the biggest drawback of heretofore known plastic brackets is the inability to reliably and consistently bond the brackets to the teeth.